Device for regenerating an optical signal, the use of such a device, and an installation including such a device

ABSTRACT

The invention relates to a device for regenerating the phase of an optical signal carrying an information encoded by phase modulation of said signal, the device comprising an optical modulation converter for converting the signal carrying the information encoded by phase modulation into at least one secondary signal carrying said information encoded by amplitude modulation, and at least one module for optically regenerating the amplitude of the secondary signal.

The present invention relates to a device for regenerating an opticalsignal, to the use of such a device, and to an installation includingsuch a device.

More precisely, the invention relates to a device for regenerating anoptical signal carrying an information encoded by phase modulation ofthe signal.

BACKGROUND OF THE INVENTION

When a signal is conveyed by an optical fiber, it is subjected tocertain kinds of distortion, such as amplitude, frequency, or phasedistortion. In order to recover a signal that is as similar as possibleto the signal as emitted, it is then necessary to pass the opticalsignal through a regeneration device.

Devices are already known in the state of the art for regenerating anoptical signal carrying information encoded by amplitude modulation ofsaid signal, e.g. by using saturable absorbers.

Unfortunately, in present-day optical transmission devices that enablerates of 40 gigabits per second (Gbits/s) or more to be obtained, everincreasing use is being made of signals that are phase-modulated, inparticular by modulation of the differential phase shift keying (DPSK)type. In that type of modulation, information is encoded in the phase ofthe signal: for example a “1” bit is encoded by inverting the phase ofthe carrier signal, while a “0” bit is encoded by a lack of phasechange.

Present optical regeneration devices do not enable signals carryinginformation encoded by phase modulation to be regenerated correctlysince such devices act only on signal amplitude. Consequently, the phasedistortion of signals which degrades the information conveyed therebycannot be eliminated or at least reduced.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to remedy that drawback by proposing asignal regeneration device that enables signals that carry informationencoded by phase modulation to be regenerated.

To this end, the invention provides a device for regenerating the phaseof an optical signal carrying an information encoded by modulating thephase of said signal, the device comprising:

an optical modulation converter for converting the signal carrying theinformation encoded by phase modulation into at least one secondarysignal carrying said information encoded by amplitude modulation; and

at least one optical amplitude regeneration module for regenerating theamplitude of the secondary signal.

The regeneration device of the invention thus makes it possible toregenerate the phase of signals carrying information encoded by phasemodulation in simple manner by using existing devices for regeneratingsignals that carry information encoded by amplitude modulation.

A regeneration device of the invention may further comprise one or moreof the following characteristics:

the regeneration device further comprises an optical modulationconverter for converting the regenerated secondary signal into a signalcarrying the information encoded by phase modulation;

the optical converter of phase modulation into amplitude modulation andthe optical converter of amplitude modulation into phase modulation arecombined as a single reversible converter;

the optical regeneration module for regenerating the secondary signalcomprises noise suppressor means;

the noise suppressor means comprise a saturable absorber;

each optical modulation converter comprises two couplers connected inseries supplying two secondary signals;

the optical amplitude regeneration module for regenerating the amplitudeof the two secondary signals comprises a single saturable absorbersimultaneously regenerating the amplitude of both secondary signals; and

the optical amplitude regeneration module for regenerating the amplitudeof the secondary signals comprises two saturable absorbers respectivelyregenerating the amplitude of each of the secondary signals.

The invention also provides an optical transmission installationincluding light signal propagation means, the installation including aregeneration device of the invention as defined above, inserted in thepropagation means.

The invention also provides the use of a device combining:

an optical modulation converter for converting a signal carrying aninformation encoded by phase modulation into at least one secondarysignal carrying said information encoded by amplitude modulation; and

at least one optical amplitude regeneration module for regenerating theamplitude of the secondary signal,

for regenerating the phase of the optical signal carrying theinformation encoded by phase modulation of the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the followingdescription given purely by way of example and made with reference tothe accompanying drawings, in which:

FIG. 1 is a block diagram of a regeneration device in a first embodimentof the invention;

FIG. 2 is a block diagram of a regeneration device in a secondembodiment of the invention;

FIG. 3 is a block diagram of a regeneration device in a third embodimentof the invention; and

FIG. 4 is a detailed diagram of a variant of a portion of theregeneration device shown in FIG. 3.

MORE DETAILED DESCRIPTION

An optical fiber given overall reference 10 is shown in FIG. 1. Thisoptical fiber is used for transmitting a light signal S carrying aninformation encoded by phase modulation of the signal. The phasemodulation used may be selected, for example, from known RZ-DPSK andNRZ-DPSK.

The information carried by the light signal S is binary information. Thebits are spaced apart in pairs by a duration T_(b) referred to as the“bit time”.

The signal S(t) is obtained by modulating the phase of a periodiccarrier signal P(t) of period T_(p). The carrier signal is selected insuch a manner that T_(b) is a multiple of T_(p).

The phase of the signal corresponding to the n^(th) bit transmittedbetween instants nT_(b) and (n+1)T_(b) is written φ(n).

The signal transmitted over the optical fiber is then written:${S(t)} = {{{{P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{(n)}}}}\quad{with}\quad n} = {E\left\lbrack \frac{t}{T_{b}} \right\rbrack}}$where E[ ] designates the integer portion function.

An optical signal regeneration device 12 is connected in series with theoptical fiber 10. The optical signal entering the regeneration device 12is written S_(e)=S, and the regenerated optical signal leaving theregeneration device is written S_(s).

The regeneration device 12 comprises a first optical modulationconverter 14 for converting the signal S_(e) carrying informationencoded by phase modulation into two signals S₁ ^(′) and S₂ ^(′)carrying said information encoded by amplitude modulation.

These two signals S₁ ^(′) and S₂ ^(′) are then regenerated using anoptical amplitude regeneration module 16.

The optical signals regenerated in the optical amplitude regenerationmodule 16 then pass through a second optical modulation converter 18 forconverting the signals carrying information encoded by amplitudemodulation into a signal S_(s) carrying the information encoded by phasemodulation. The signal S_(s) thus carries the same information encodedby phase modulation as the signal S_(e), but it has in the meanwhilebeen regenerated by the optical amplitude regeneration module 16.

It can be seen that within the regeneration device 12, the signal S_(e)for regeneration is split into two signals. Reference is then made tothe two arms of the regeneration device 12 to refer to the two pathstaken by the signals. The signal transmitted over the first arm (top armin FIG. 1) are given the index 1, whereas the signals conveyed over thesecond arm (bottom arm) are given the index 2.

Each optical modulation converter 14, 18 is a DPSK demodulatorimplemented by means of two 3 decibel (dB) couplers 20, 22 for themodule 14, and 26 and 28 for the module 18, together with a respectivedelay element 24 or 30, interposed between the two couplers of the firstarm.

A 3 dB coupler is a passive optical quadripole. The two poles via whichthe signals enter are written Inlet₁ and Inlet₂ and the two poles viawhich the signals leave are written Outlet₁ and Outlet₂. These poles arethen related by the following relationship: $\begin{pmatrix}{Outlet}_{1} \\{Outlet}_{2}\end{pmatrix} = {{{\begin{pmatrix}\alpha & {\alpha.{\mathbb{e}}^{{\mathbb{i}} \cdot \frac{\pi}{2}}} \\{\alpha.{\mathbb{e}}^{{\mathbb{i}}.\frac{\pi}{2}}} & \alpha\end{pmatrix} \cdot \begin{pmatrix}{Inlet}_{1} \\{Inlet}_{2}\end{pmatrix}}\quad{with}\quad\alpha} = \frac{\sqrt{2}}{2}}$

Since a coupler is a passive component, the two inlet poles can beinterchangeably the left or the right poles without modifying the aboverelationship. A 3 dB coupler is said to be “reversible”.

The first 3 dB coupler 20 of the optical modulation converter 14 has asits sole input signal the signal S_(e) entering via the inlet 1. Theinlet 2 is connected to ground. The signal obtained at the outlet 1 ofthe 3 dB coupler 20 is written S₁ and the signal obtained at the outlet2 of the 3 dB coupler 20 is written S₂.

These two signals S₁ and S₂ can then be expressed as follows:${S_{1}(t)} = {{\alpha \cdot {S_{e}(t)}} = {\frac{\sqrt{2}}{2}{{P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{(n)}}}}}}$${S_{2}(t)} = {{\alpha \cdot {\mathbb{e}}^{{\mathbb{i}}\quad\frac{\pi}{2}} \cdot {S_{e}(t)}} = {\frac{\sqrt{2}}{2}{{P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {({{\varphi{(n)}} + \frac{\pi}{2}})}}}}}$

The delay element 24 disposed in the first arm of the optical modulationconverter 14 between its two 3 dB couplers 20 and 22 serves to delay thesignal S₁ by one bit time T_(b) before it enters into the second 3 dBcoupler 22. The delayed signal is then written:${S_{1}\left( {t - T_{b}} \right)} = {{{\alpha \cdot {P\left( {t - T_{b}} \right)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{(n^{\prime})}}}}\quad{where}\quad n^{\prime}} = {{E\left\lbrack \frac{t - T_{b}}{T_{b}} \right\rbrack} = {{E\left\lbrack {\frac{t}{T_{b}} - 1} \right\rbrack} = {n - 1}}}}$

The signal S₂ obtained on the second arm at the outlet from the first 3dB coupler 20 enters the second inlet of the second 3 dB coupler 22directly.

The outlet from the second 3 dB coupler 22 then deliver signals S₁ ^(′)and S₂ ^(′) which are written as follows:${S_{1}^{\prime}(t)} = {{\alpha \cdot \left( {{S_{1}\left( {t - T_{b}} \right)} + {{\mathbb{e}}^{{\mathbb{i}} \cdot \frac{\pi}{2}} \cdot {S_{2}(t)}}} \right)} = {\frac{1}{2} \cdot {P(t)} \cdot \left( {{\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 1})}}} - {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{(n)}}}} \right)}}$${S_{2}^{\prime}(t)} = {{\alpha \cdot \left( {{{\mathbb{e}}^{{\mathbb{i}} \cdot \frac{\pi}{2}} \cdot {S_{1}\left( {t - T_{b}} \right)}} + {S_{2}(t)}} \right)} = {\frac{1}{2} \cdot {P(t)} \cdot \left( {{\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 1})}}} + {\mathbb{e}}^{i \cdot {\varphi{(n)}}}} \right) \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot \frac{\pi}{2}}}}$since T_(b) is a multiple of the period T_(p) of P, P(t−T_(b))=P(t).

The modulation used for encoding information on the carrier signal S_(e)is two-state differential phase keying. The two states selected for thephase φ are 0 and π. Two successive bits are thus encoded by signals ofphases that are either equal or opposite.

Consequently, if φ(n)=φ(n−1), then the signal S₁ ^(′) is zero and thesignal S₂ ^(′) is non-zero. Similarly, if φ(n)=φ(n−1)+π, then the signalS₁ ^(′) is not zero while the signal S₂ ^(′) is zero. The signals S₁^(′) and S₂ ^(′) thus carry information that is encoded by amplitudemodulation.

The optical amplitude regeneration module 16 of the regeneration device12 enables these two signals S₁ ^(′) and S₂ ^(′) to be regenerated. Thismodule comprises noise suppression means 17. The noise suppression means17 used are, for example, saturable absorbers that eliminate the noisefrom the two signals S₁ ^(′) and S₂ ^(′) Any other device forregenerating a signal carrying information encoded in amplitude couldalso be used.

The signal obtained at the outlet from the first arm of the first 3 dBcoupler 26 of the second optical modulation converter 18 is written S₁^(″). This signal S₁ ^(″) is equal to:${S_{1}^{''}(t)} = {{{\alpha \cdot {S_{1}^{\prime}(t)}} + {\alpha \cdot {\mathbb{e}}^{{\mathbb{i}}\frac{\pi}{2}} \cdot {S_{2}^{\prime}(t)}}} = {{- \frac{\sqrt{2}}{2}} \cdot {P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{(n)}}}}}$

The signal S₁ ^(″ ″) corresponds to the signal S₁ ^(″) after it haspassed through a delay element 30, giving:${S_{1}^{\prime\prime\prime}(t)} = {{- \frac{\sqrt{2}}{2}} \cdot {P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 1})}}}}$

The signal obtained at the outlet from the second arm of the first 3 dBcoupler 26 of the second optical modulation converter 18 is written S₂^(″ ′):${S_{2}^{\prime\prime\prime}(t)} = {{{\alpha \cdot {\mathbb{e}}^{{\mathbb{i}}\frac{\pi}{2}} \cdot {S_{1}^{\prime}(t)}} + {\alpha \cdot S_{2}^{\prime}}} = {\frac{\sqrt{2}}{2} \cdot {P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 1})}}} \cdot {\mathbb{e}}^{{\mathbb{i}}\quad\frac{\pi}{2}}}}$

These two signals S₁ ^(″ ′) and S₂ ^(″ ′) are then inserted into thesecond 3 dB coupler 28 of the second optical converter 18, having itsoutlet 2 connected to ground and the signal S_(s) obtained from theoutlet 1 of the second 3 dB coupler 28 then has the value:${S_{s}(t)} = {{{\alpha \cdot {S_{1}^{\prime\prime\prime}(t)}} + {\alpha \cdot {\mathbb{e}}^{{\mathbb{i}}\quad\frac{\pi}{2}} \cdot {S_{2}^{\prime\prime\prime}(t)}}} = {{- {P(t)}} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 1})}}}}}$

It can be seen that the signal S_(s) corresponds to the signal S_(e),offset by one bit time, and ignoring sign.

This thus provides a device for regenerating the light signal S carryinginformation encoded by modulating the phase of said signal. Theregeneration device presents the advantage of comprising passiveelements only.

A second embodiment shown in FIG. 2 uses the reversibility property of 3dB couplers. Since the optical modulation converters 14 and 18 describedabove are symmetrical devices using reversible components, these opticalmodulation converters are likewise reversible devices. The same resultsare obtained regardless of the direction light travels through them.

The term “forward” direction is used for travel from left to right, andthe term “reverse” direction is used for travel from right to left.

The regeneration device 12 has a single optical modulation converter 14connected in series with an optical amplitude regeneration module 16comprising noise suppressor means 17.

The signals traveling in the forward direction, obtained at the outletfrom the optical amplitude regeneration module 16 are identical to thoseobtained at the outlet from the same module in the first embodiment. Areflector module 32 is disposed at the outlet from the optical outletregeneration module 16. The reflector module 32 comprises two mirrors 34and 36 each placed on a respective arm of the regeneration device 12.

It is assumed that the path lengths traveled between the two outletsfrom the 3 dB coupler 22 and the mirrors 34 and 36 of the reflectordevice 32 are equal so that the phase difference between the signals isconserved. Otherwise, it is necessary to insert an element that inducesa phase difference for adjustment in order to compensate for the phasedifference between the two paths.

The signals obtained after reflection on each of the mirrors 34 and 36of the reflection device 32 are then transmitted in the reversedirection towards the optical modulation converter 14 after passing asecond time through the optical amplitude regeneration module 16.

The equations for the signals S₁, S₁ ^(′), S₁ ^(″), S₁ ^(″ ′), S₂, S₂^(′), S₂ ^(″), and S₂ ^(″ ′) marked in FIG. 2 are the same as theequations for the embodiment shown in FIG. 1.

In this embodiment, the regeneration device 12 also includes an opticalcirculator 40 enabling it to be inserted in the middle of the opticalfiber 10. The signal S_(e) traveling along the optical fiber 10penetrates into the circulator 40 via a first port V₁ and leaves via asecond port V₂ going towards the optical modulation converter 14. Afterbeing regenerated, the signal passes back into the optical circulator 40via the second port V₂ and leaves via a third port written V₃ in theform of the regenerated outlet signal S_(s),

A third embodiment of the invention is shown in FIG. 3. Thisregeneration device 12 has a single optical modulation converter 42 andan optical amplitude regeneration module 44 comprising noise suppressormeans 17 as in the second embodiment.

The optical modulation converter 42 has two 3 dB couplers 54 betweenwhich there are placed, on the first arm, a one bit delay element sothat the signals traveling in the forward direction are subjected to thesame modifications as in the second embodiment.

The optical conversion of the modulation of the signal carryinginformation encoded by phase modulation into two signals carryinginformation encoded by amplitude modulation thus takes place in a manneridentical to that of the preceding embodiment. The expressions for thesignals S₁, S₁ ^(′), S₂, and S₂ ^(′), are unchanged, giving:$S_{1}^{\prime} = {{\alpha \cdot \left( {{S_{1}\left( {t - T_{b}} \right)} + {{\mathbb{e}}^{{\mathbb{i}} \cdot \frac{\pi}{2}} \cdot {S_{2}(t)}}} \right)} = {\frac{1}{2} \cdot {P(t)} \cdot \left( {{\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 1})}}} - {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{(n)}}}} \right)}}$${S_{2}^{\prime}(t)} = {{\alpha \cdot \left( {{{\mathbb{e}}^{{\mathbb{i}} \cdot \frac{\pi}{2}} \cdot {S_{1}\left( {t - T_{b}} \right)}} + {S_{2}(t)}} \right)} = {\frac{1}{2} \cdot {P(t)} \cdot \left( {{\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 1})}}} + {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{(n)}}}} \right) \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot \frac{\pi}{2}}}}$

The signals S₁ ^(′) and S₂ ^(′) obtained respectively at the outletsfrom the first and second arms of the optical modulation converter aresubsequently transmitted to the optical amplitude regeneration module44.

Unlike the second embodiment described above, the signal S₁ ^(′), onceregenerated, is reinjected into the optical modulation converter via theinlet on the second arm, and the signal S₂ ^(′), once regenerated, isreinjected into the optical modulation converter via the inlet in thefirst arm.

By means of this embodiment, there is no longer any need to use areflection module. This assumes that the amplitude regenerator means 46performs its function in both directions and simultaneously. Onepossible practical embodiment thereof would be a saturable absorber ofarea that is sufficiently large to enable it to process both signalssimultaneously.

While the signals S₁ ^(′) and S₂ ^(′) are traveling in the reversedirection through the optical modulation converter 42, the signalsobtained at the outlet from the second 3 dB coupler 54 are written asfollows:${S_{1}^{''}(t)} = {{{\alpha \cdot {S_{2}^{\prime}(t)}} + {\alpha \cdot {\mathbb{e}}^{{\mathbb{i}}\frac{\pi}{2}} \cdot {S_{1}^{\prime}(t)}}} = {\frac{\sqrt{2}}{2} \cdot {P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}}\quad\frac{\pi}{2}} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 1})}}}}}$${S_{2}^{\prime\prime}(t)} = {{{\alpha \cdot {\mathbb{e}}^{{\mathbb{i}}\frac{\pi}{2}} \cdot {S_{2}^{\prime}(t)}} + {\alpha \cdot S_{1}^{\prime}}} = {{- \frac{\sqrt{2}}{2}} \cdot {P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{(n)}}}}}$

After passing through the one bit delay element, the signal S₁ ^(″)becomes a signal S₁ ^(′ ″) having the following expression:${S_{1}^{\prime\prime\prime}(t)} = {\frac{\sqrt{2}}{2} \cdot {P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot \frac{\pi}{2}} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 2})}}}}$

To obtain results that are substantially the same as in the secondembodiment, a two bit time delay element is placed in the second arm.

This delay element must act only on signals traveling along the secondarm in the reverse direction.

For this purpose, two optical circulators 48 are used enabling thesignal transmitted along the second arm in the forward direction toremain unchanged, while causing the signals transmitted along the secondarm in the reverse direction to pass through the two bit time delayelement 50.

After passing through the two bit delay element 50, the signal S₂ ^(″)becomes the signal S₂ ^(′ ″) having the following expression:${S_{2}^{\prime\prime\prime}(t)} = {{S_{2}^{\prime\prime}\left( {t - {2 \cdot T_{b}}} \right)} = {{- \frac{\sqrt{2}}{2}} \cdot {P(t)} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 2})}}}}}$

These two signals S₁ ^(′ ″) and S_(s) ^(′ ″) then penetrate into thefirst 3 dB coupler 54 whose outlet in the second arm in the reversedirection then gives:${S_{s}(t)} = {{{\alpha \cdot {\mathbb{e}}^{{\mathbb{i}}\quad\frac{\pi}{2}} \cdot {S_{1}^{\prime\prime\prime}(t)}} + {\alpha \cdot {S_{2}^{\prime\prime\prime}(t)}}} = {{- {P(t)}} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {\varphi{({n - 2})}}}}}$

It can thus be seen that the signal S_(s) is equal to the signal S_(e)offset by two bit times, ignoring sign.

The optical regeneration module 44 that is used in the third embodimentshown in FIG. 3 and that operates simultaneously in both directions canalso be made by using two saturable absorbers.

This module, shown in greater detail in FIG. 4, has first and secondnoise suppressor means 58 and 60 and two optical circulators 62 and 64.The noise suppressor means 58 and 60 are constituted, for example, bysaturable absorbers. By means of the two optical circulators, thesignals traveling from the first arm to the second arm of the opticalmodulation converter 42 pass through the first saturable absorber 58,while the signals traveling in the opposite direction pass through thesecond saturable absorber 60.

This optical regeneration device is thus entirely optical and thereforepassive. It enables an optical signal carrying information encoded byphase modulation of the signal to be regenerated.

1. A device for regenerating the phase of an optical signal carrying aninformation encoded by modulating the phase of said signal, the devicecomprising: an optical modulation converter for converting the signalcarrying the information encoded by phase modulation into at least onesecondary signal carrying said information encoded by amplitudemodulation; at least one optical amplitude regeneration module forregenerating the amplitude of the secondary signal; and an opticalmodulation converter for converting the regenerated secondary signalinto a signal carrying the information encoded by phase modulation.
 2. Aregeneration device according to claim 1, in which the optical converterof phase modulation into amplitude modulation and the optical converterof amplitude modulation into phase modulation are combined as a singlereversible converter.
 3. A regeneration device according to claim 1, inwhich the optical regeneration module for regenerating the secondarysignal comprises noise suppressor means.
 4. A regeneration deviceaccording to claim 3, in which the noise suppressor means comprise asaturable absorber.
 5. A regeneration device according to claim 1, inwhich each optical modulation converter comprises two couplers connectedin series supplying two secondary signals.
 6. A regeneration deviceaccording to claim 5, in which the optical amplitude regeneration modulefor regenerating the amplitude of the two secondary signals comprises asingle saturable absorber simultaneously regenerating the amplitude ofboth secondary signals.
 7. A regeneration device according to claim 5,in which the optical amplitude regeneration module for regenerating theamplitude of the secondary signals comprises two saturable absorbersrespectively regenerating the amplitude of each of the secondarysignals.
 8. An optical transmission installation comprising opticalsignal propagation means, the installation including a regenerationdevice according to claim 1 inserted in the propagation means.
 9. Theuse of a device combining: an optical modulation converter forconverting a the signal carrying an information encoded by phasemodulation into at least one secondary signal carrying said informationencoded by amplitude modulation; at least one optical amplituderegeneration module for regenerating the amplitude of the secondarysignal; and an optical modulation converter for converting theregenerated secondary signal into a signal carrying information encodedby phase modulation; for regenerating the phase of the optical signalcarrying the information encoded by modulating the phase of said signal.